Accelerators are becoming increasingly important in medicine, clean energy and national security. Accelerators can be used for safer nuclear reactors, industrial irradiation, cancer therapy and cargo inspection.
A strong economic consideration is accelerator footprint; synchrotrons and linacs typically reserve much larger civil accommodations. With its compact footprint and continuous beam current, both high and low intensity, the cyclotron is the current industrial and medical standard. A new player to advanced accelerator applications is the Fixed Field Alternating Gradient Accelerator (FFAG), including a scaling FFAG version and a more recent invention, a non-scaling FFAG.
The so-called scaling FFAG (either spiral or radial-sector FFAGs) is characterized by geometrically similar orbits of increasing radius. Direct application of high-order magnetic fields and edge focusing maintains a constant tune and optical functions during the acceleration cycle and avoids low-order resonances. In general, scaling FFAG designs are not compatible with isochronous orbits and therefore not compatible with CW operation.
The non-scaling FFAG was subsequently developed (C. Johnstone, et al., “Fixed Field Circular Accelerator Designs”, PAC '99, New York, P. 3068.). The non-scaling FFAG was proposed for muon acceleration and utilized simple, combined function magnets like a synchrotron. However, it did not maintain a constant tune and was not suitable for an accelerator with a modest RF system and a slower acceleration cycle.
An initial approach to tune stabilization in a linear-field non-scaling FFAG was developed (See U.S. Pat. No. 7,880,146 B2; “Tune-stabilized, non-scaling, fixed-field, alternating gradient accelerator”, Johnstone, Carol J.) in which a set of seven equations relate a number of parameters specifying focus and defocus magnets together. A linear field condition was assigned in order to stabilize machine tune, but given the linear condition, compact stable orbits cannot be achieved at near relativistic and relativistic energies.
Further, U.S. Publication No. 2012/013274 describes a Non-Scaling FFAG Accelerator design, wherein the linear field condition is removed in order to realize advanced machine properties and optimal designs. The nonlinear field condition, or high-order field, allows for more constant machine tune as a function of momentum or energy and more compact machines resulting in smaller apertures. Limits were set on the extraction and injection radii for compact machines in an optimizer search for stable solutions, but this is a constraint that does not reflect fundamental dynamics unlike limits required on cell phase advance or tune. These additional conditions were imposed only to guide the optimizer search but do not comprise a solution.
With the cyclotron as the current industrial and medical standard, a competing CW FFAG could potentially have a broad impact on medical accelerators, proton drivers for neutron production, accelerator-driven nuclear reactors, accelerator transmutation of waste (ATW), and production of radiopharmaceuticals, as well as open up a range of as-yet unexplored industrial applications.